1. Field of the Invention
The present invention relates to bottom layer anti-reflective compositions for multilayer photoresist systems, and particularly thermosetting resins employed in such anti-reflective compositions.
2. Background of the Prior Art
Bottom layer anti-reflective coating compositions ("ARCs") for use in multilayer photoresist systems typically contain high molecular weight thermoplastic binders such as polysulfones and polyimides with high differential solubility, to avoid intermixing with the top layer photoresist. These thermoplastic bound ARCs necessarily use solvents such as N-methylpyrrolidone (NMP), gamma-butyrolactone and THFA (Tetrahydrofurfuyl Alcohol), which are hygroscopic, have high surface tensions and exhibit low volatility. While these properties may aid differential solubility, there are drawbacks. That is, a variety of film defects such as dimpling, dewetting, voiding, bubbling, and thickness variations can result from the use of low volatility solvents.
With the trend toward sub-half-micron feature sizes, new inadequacies of thermoplastic ARC binders themselves have also become apparent. The most notable of these is their incomplete resistance to intermixing with photoresists (even when using low volatility solvents). Slight intermixing always occurs producing a small but discernable distortion at the bottom of resist features. Because the feature sizes are so small, even this slight distortion is unacceptable for practical device manufacturing.
In order to overcome these drawbacks, there has arisen a need to develop binders for ARCs from thermosetting polymers coatable from fast-drying solvents so that solvent resistance and coating quality can be improved. Two component thermosetting polymer systems such as hydroxyl-functional resins used in conjunction with aminoplast crosslinking agents (hereinafter referred to as HA coatings) are known to be soluble in highly volatile organic solvents and have been used in a wide range of conventional industrial coatings, including modern photoresist compositions. They, therefore, represent highly desirable alternatives to thermoplastic, high molecular weight polymer binders.
However, until now, HA coatings have been characterized by properties which inhibit their use in bottom layer ARCs, e.g., (1) insufficient optical density in ultra thin films; (2) poor photoresist-solvent resistance; (3) storage instability in catalyzed form; (4) commercially unfeasible synthesis techniques for linking chromophores; and (5) inherently restrictive deep ultraviolet absorption, so as to be nonfunctional for g-line and I-line.
In the past, HA coatings, when employed in microlithographic compositions, have been employed primarily as single layer photoresist. These photoresist contain photoacid generators which delay acid-catalyzed crosslinking until exposure to light. Although such delayed catalysis is advantageous in stabilizing a top layer HA coating against gelation during storage, the delay is a drawback to the use of HA coatings as bottom-layer ARCs. That is, since bottom-layer ARCs are not exposed to light until after a layer of photoresist is overcoated onto the ARC, a bottom-layer ARC made from HA coatings must a fortiori cure and gel from an active catalyst prior to exposure or risk nonadherence to the substrate.
Also, the prior art HA compositions provide little, if any, ultraviolet light absorption.
Examples of the above-described prior art HA-compositions are disclosed in U.S. Pat. Nos. 4,341,859; 4,478,932; 4,518,676; 4,734,444; 5,034,304; and 5,376,504.
U.S. Pat. Nos. 3,744,904 and 4,149,888 do disclose photomasks made by physically admixing (without reacting) HA coatings and ultraviolet light absorbers such as azo/diazonium coupling products. However, such coatings are still incapable of imparting the degree of optical density or the level of ultraviolet light absorption required at typical ARC thicknesses, i.e., about 500-2500 .ANG.. Such coatings could be more heavily dye-loaded to increase their optical density but this would prevent the requisite solvent resistance needed to inhibit intermixing with the top photoresist layers once the photoresist is applied over the ARC and baked at typically 90.degree.-140.degree. C.
European patent application 0636941 does disclose a deep ultraviolet thermosetting ARC, but its synthesis is unfeasibly complex, requiring retention of a poly(epoxide) molecular form throughout synthesis, and their reactivity is restricted to deep ultraviolet chromophores. The crosslinking agents are polyfunctional phenolic compounds having a substituted anthracene moiety as the structural center. Although functional under ideal conditions, these ARCs have limited commercial applicability for several reasons. First, preparation of the phenolic/anthracine crosslinking agents involves many steps, including chromatographic purifications of intermediates, making the compounds unrealistically expensive to produce. Secondly, the optical densities of the coating products are low in comparison to the values of 7-15/micron generally accepted as necessary for advanced deep UV lithography.sup.1. The film transmissivity data provided for 1000 .ANG.-thick coatings prepared in Examples 1 and 2 of E.P. 0636941, for example, correspond to optical densities (at deep U.V. 248 nm) of only 6.8/micron and 3.1/micron, respectively. While it is likely that the optical density of the coatings could be improved by increasing the ratio of the crosslinking agent to the epoxy resin in the compositions, such formulation would risk reducing resist compatibility (i.e., increase intermixing). Furthermore, because the crosslinking agents are largely comprised of aromatic and polycyclic aromatic compounds, increasing their concentration would reduce the etch rate of the ARC relative to the photoresist, leading to negative etch biasing and loss of dimensional control. FNT .sup.1 J. Sturdevant, M. Chaara, R. Elliot, L. Hollifield, R. Soper, D. Stark, N. Thane, J. Petersen, "Antireflection Coating Process Characterization and Improvement for DUV Lithography at 0.25 .mu.m,", SPIE Proceedings: 1995 International Microlithography Symposium, Feb. 19-24, 1995.
It is therefore a principal object of the present invention to provide a novel thermosetting ARC and method for making which negates the drawbacks of the prior art.
It is a particular object of this invention to provide a high optical density ARC, resistant to photoresist intermixture at ultra-thin layer thicknesses.
It is an additional object of the present invention to provide a method for making improved thermosetting ARCs for a broad range of exposure wavelengths including deep ultraviolet, I-line and g-line.
It is also an object of the present invention to provide an improved HA coating for use as a bottom-layer ARC which can be stably stored in solution, yet with an active catalyst, so as to crosslink after removed from storage but prior to light exposure.